Machine operator altered shift (7)
I believe the answer is:
treadle
'machine operator' is the definition.
(I've seen this before)
'altered shift' is the wordplay.
'shift' indicates an anagram.
'altered' is an anagram of 'TREADLE'.
(Other definitions for treadle that I've seen before include "Rocking lever operated by foot" , "Bar" , "Foot lever powering machine" , "Rocker in a factory" , "Alerted (anag)" .)